The present invention provides a new process for making hollow spheres, particularly hollow macrospheres (the term "macrosphere" is used herein for spheres that are larger than about 500 micrometers (1/2 millimeter) in diameter, and contrasts with the term "microsphere," which is generally applied to smaller-sized spheres best viewed with a microscope).
In addition, hollow spheres made by the new process are unique in structure and properties, adapting them to a wide variety of uses.
An example of the advance made by the invention lies in the use of hollow spheres as lightweight filler materials. At the present time, the lowest-cost commercial hollow spherical filler materials are microspheres on the order of 5 to 300 micrometers in diameter. These commercial hollow microspheres may be either glass microspheres (made by passing particles of amorphous glass or glass-forming ingredients that contain a blowing agent through a current of heated air or a flame; see Beck et al., U.S. Pat. No. 3,365,315 and Veatch et al., U.S. Pat. No. 3,030,215) or organic polymeric microspheres (made by spray-drying liquid droplets that contain a film-forming material and a blowing agent dissolved in a volatile solvent; see Veatch et al., U.S. Pat. No. 2,797,201; or by forming an aqueous dispersion of organic polymerizable monomeric materials and a liquid blowing agent, then polymerizing the monomeric material to form a thermoplastic expandable polymer particle having encapsulated therein the volatile liquid blowing agent, and then heating the particle; see Morehouse et al., U.S. Pat. No. 3,615,972).
While the commercial hollow microspheres are useful in many ways, only larger-diameter hollow spheres can satisfy other important uses. For example, larger hollow spheres are needed for use with smaller sized hollow spheres in bi-or tri-modal packing arrangements (see Beck, U.S. Pat. No. 3,585,157) to achieve filled articles having a lower density than articles filled only with the smaller spheres. One large potential use for such bi- or tri-modal filling would be in syntactic foams, which have been the subject of extensive development effort to provide flotation materials useful in off-shore oil-drilling rigs and other water-borne vessels.
The potential utility for larger hollow spheres has been recognized for a long time. But until the present invention, no one has apparently known how to make such spheres at low cost. Matthews et al., U.S. Pat. No. 3,838,998 teaches a process said to prepare moderately sized glass spheres (50 to 5000 micrometers in diameter), but the spheres prepared are not as large as desired and the process has never resulted in commercial use. Insofar as I am aware, the only large spheres that have been commercially available -- epoxy spheres on the order of 0.1 to 1 centimeter in diameter apparently made by individually coating spherical polystyrene foam particles with a curable epoxy-based composition and then curing the coating; or composite spheres on the order of a centimeter in diameter made by separately molding hemispheres and bonding them together -- have been too high in cost for widespread utility. Despite the commercial availability of such larger spheres for many years, and despite the recognized potential utility for such spheres if they could be provided at lower cost, no one has previously provided a low-cost large-diameter hollow sphere.